Hi,
Yes, zaresa is right, but if you have many mailboxes this is not best practis, and it does not solve new mailboxes.
On the Site level "green globe icon" select permissions.
Add youre account and grant it permissions.
Note:
"Permissions admin" can not read mailboxes.
"User" can read mailboxes.
"service account" can do everything, good and bad.
Good Luck.
Wibbe